In the oil and gas industry, wells are drilled to access subsurface hydrocarbon-bearing rock formations. Once the appropriate apparatus is in place in the well and at surface, hydrocarbons may flow from the formation or reservoir to surface. However, over a period of time conditions may change such that the rate of production of hydrocarbons falls or even stops. For example, in gas-producing wells, as the reservoir pressure drops, the velocity of the gas flowing through the production tubing may become insufficient to transport liquids present in the gas to surface. Thus, over time, liquids accumulate in the well and ultimately hinder production.
One established remedial treatment for this situation involves installing a smaller diameter tubing or string inside the existing production tubing, such that the gas travels to surface though the smaller diameter tubing. The reduction in flow area results in an increase in flow velocity sufficient to carry liquids from the wellbore, hence the use of the term “velocity string” to describe the tubing.
Oil and gas wells feature multiple safety systems to prevent uncontrolled release of fluid from the reservoir, including the provision of one or more safety valves in the production tubing which carries the oil or gas to surface. A typical safety valve will be mounted inside the production tubing and will be controllable from surface via one or more hydraulic control lines mounted on the outside of the production tubing. The valve may be a spring-biased flapper valve which, when activated, acts as a check valve and will open to permit flow from surface into the well but will prevent flow from the reservoir to surface.
When a velocity string is installed in a well it is desirable to retain the functionality of the safety valve. Thus one solution is to provide a velocity string dimensioned such that the upper end of the string terminates just below the existing safety valve. The string may be suspended from a packer which is run in and set below the safety valve. If desired, a further velocity string is positioned in an upper section of the well, the lower end of this string terminating just above the safety valve. However, this arrangement has a number of disadvantages, one being the discontinuity in the flow area between the ends of the velocity strings: as the flow leaves the upper end of the lower string the gas velocity will fall, and liquid may drop out of the fluid stream. The presence of the packer also tends to reduce the available flow area.
In some cases it may not be possible to run an appropriate packer through the restriction created by the safety valve. In this situation a velocity string may be provided which passes through the safety valve; however this eliminates the functionality of the safety valve, removing one layer of safety from the well operations.